Composite video signals of the type used in television systems include video and synchronizing information. The video information is divided into sequential trace and retrace signals with the synchronizing information transmitted as pulses during the retrace portion of the video information. During the trace interval, the video signal varies between black and white levels. The retrace interval video signal includes a blanking portion which is at or slightly greater than black level so that the electron beam in the cathode ray tube (CRT) is turned off during retrace. If all of the levels of the various components of the video signal are properly adjusted at the studio and properly transmitted and received, the video signals can be properly displayed on a CRT screen. Unfortunately, adjustment errors and transmitting and receiving errors require processing and correction of the video signal to achieve a satisfactorily displayed image.
Some of the more common errors in the received signal include improper black and white levels and improper synchronizing pulse amplitude. Since the automatic gain control in typical television receivers sets-up on the synchronizing pulse amplitude, improper synchronizing pulse amplitude can cause the received signal to be amplified more or less than desired. If the black level of the trace interval signal is set improperly with respect to the blanking level, the displayed image will be too dark or too light depending upon the direction of the error. Similarly, an improperly set white level will result in improper contrast assumming black level is set correctly. In typical television receivers, the contrast and brightness controls can be used to correct, at least in part, for such errors. Adjustment of the contrast and brightness controls to correct erroneous composite video signals, however, cannot correct all errors and necessitates frequent readjustment to provide a proper displayed image.
The signals received by a television receiver are typically provided by a variety of sources each of which can have different errors in signal levels necessitating readjustment of the contrast and brightness controls to produce a properly displayed image. For example, when channels are changed, any signal level errors will necessarily be different. Also, on the same channel the signal may be provided by a variety of sources in the studio such as different cameras, video tape, movie film, and the like. Since each source can have its own unique characteristics, switching from one source to another can alter the signal levels. Compensation for the different signal levels of the various studio signal sources is generally incomplete. Accordingly, the composite video signal received by a television receiver may also include undesired variations in signal levels.
Prior art television receivers have included various forms of automatic control circuitry in an attempt to compensate for such undesired contrast and brightness variations. One prior art form of black level control includes the so-called "back porch clamp" which clamps the black level of the video signal to the blanking level. If the black level of the video signal is improperly set with respect to the blanking level, however, this form of automatic black level control does not correct the signal properly.
Another form of automatic contrast and brightness control involves detecting and clamping to the blackest and whitest portions of the received signal. While such circuitry circumvents the primary disadvantages of back porch clamping, under some signal conditions the displayed image is improper. For example, on scenes where there is no black in the image, the control circuitry can improperly clamp to a portion of the image not intended to be black and cause those portions to be black. Similarly, in a dark scene with no white level signals intended, the lightest part of the image can be clamped to white level thereby distorting the intended and desired image. Control circuitry of this type generally also requires peak detecting and sample and hold circuits. Since such circuits will tend to set-up on blanking interval signals, a blanking generator which anticipates the blanking intervals is necessary. Providing such circuitry requires substantial expense and circuit complexity for proper operation.
Other forms of automatic contrast and brightness control circuitry have been used; however, known prior art circuitry of this type, including the types mentioned above, has numerous deleterious effects. For example, the compensation for video signal errors can be incomplete or incorrect under varying signal conditions. Additionally, some forms of prior art circuitry are unduly complex and expensive or require compromises which deleteriously effect the displayed image.
Prior art television receivers commonly include beam current or brightness limiting circuitry. Such circuitry typically detects beam currents in the cathode ray tube in excess of a threshold and limits the video signal drive to the cathode ray tube in response to the excessive beam currents. Typically, such circuitry detects the excessive beam currents by detecting the current on the low voltage or return side of the voltage multiplier which provides the high voltage to the cathode ray tube. Such prior art beam current limiters, however, merely limit excessive beam current. If the beam current is below the threshold, the beam current limiter does not control the video signal.